The present invention relates to magnetic bearings and, more particularly, to a system and method for controlling an active magnetic bearing system for use in various applications, including satellites and other space applications. The magnetic bearing system is controlled using a combination of modal transformations and steady-state gains that vary continuously and non-linearly with the rotational speed of the rotor being supported by the magnetic bearing system.
Magnetic bearings have been used to suspend a rotational body, such as a rotor, with magnetic force in a non-contact fashion. That is, instead of physically supporting the rotor using lubricated bearings that physically contact the rotor, various magnets are spaced radially around the rotor and their magnetic forces suspend the rotor without any physical contact. In order to provide stable support for the rotor, the magnetic bearing suspends the rotor within five degrees-of-freedom.
Generally, there are two categories of magnetic bearings, passive magnetic bearings and active magnetic bearings. Passive magnetic bearings are the simplest type, and use permanent magnets or fixed strength electromagnets to support the rotor. Thus, the properties of the bearing, such as the magnetic field strength, may not be controlled during operation. Conversely, active magnetic bearings are configured such that the magnetic field strength of the bearing is controllable during operation. To accomplish this, at least one active magnetic bearing channel may be provided for each degree-of-freedom of the shaft. An active magnetic bearing channel may include a position sensor, a controller operating according to a predetermined control law, and an electromagnetic actuator. In general, the position sensor senses the position of the shaft and supplies a signal representative of its position to the controller. The controller, in accordance with the predetermined control law, then supplies the appropriate current magnitude to the electromagnetic actuator, which in turn generates an attractive magnetic force to correct the position of the shaft.
The control law in an active magnetic bearing system channel may be implemented using a multi-order filter. In order to provide stable, high response control of the rotor, the filter characteristics may need to vary as the rotational speed of the rotor varies. This may be the case if the rotor experiences gyroscopic effects, which increases with rotational speed. One method of varying filter characteristics is through the use of gain scheduling, in which various filters are switched in and out of operation at predetermined rotational speed setpoints. With gain scheduling, the various filters operate optimally at the middle of their predetermined speed ranges, are off-design near the boundaries, and transition through a discontinuity at the switch points. Another method of varying filter characteristics is to implement a continuous, linear gain change of one or more filters as a function of speed. This method provides stable control, but is less than optimal over the entire operating speed range of the rotor. Yet another method of varying filter characteristics is to change all of the coefficients of the filters in each channel as a continuous function of speed. This method, however, is computationally intensive.
Hence, there is a need for a system and method for controlling an active magnetic bearing that addresses one or more of the above-noted drawbacks. Namely, a system and method of control that is continuous over the operating speed range of the rotor, and/or is optimal over the operating speed range, and/or is not computationally intensive.
The present invention provides system and method for controlling an active magnetic bearing assembly by varying the system control laws in a continuous non-linear fashion as a function of rotor rotational speed.
In one embodiment of the present invention, and by way of example only, an active magnetic bearing system for rotationally suspending a rotor having a center of gravity includes at least two displacement sensors, a rotational speed sensor, a controller, and at least two electromagnetic actuators. Each displacement sensor is operable to sense rotor displacements in a sensor frame of reference that is displaced from the rotor center of gravity and supply displacement signals representative thereof. The rotational speed sensor is operable to sense a rotational speed of the rotor and supply a rotational speed signal. The controller is coupled to receive the displacement signals from each of the displacement sensors and the rotational speed signal from the rotational speed sensor. The controller, in response to the received signals, is operable to transform the displacement signals from the sensor frame of reference to a center of gravity frame of reference, generate correction signals according to predetermined gains to eliminate first and second linear displacements of the rotor""s center of gravity and first and second rotational displacements around the rotor""s center of gravity, vary the predetermined gains of at least the correction signals generated to eliminate the rotational displacements in a continuous non-linear fashion based on the received rotational speed signal, and transform the correction signals to an actuator frame of reference that is displaced from the rotor center of gravity. Each electromagnetic actuator is coupled to receive the transformed correction signals and is operable, in response thereto, to eliminate the sensed rotor displacements.
In another embodiment of the present invention, in a system including at least a rotor having a center of gravity, and an active magnetic bearing system having a plurality of displacement sensors each sensing rotor displacements according to sensor coordinates, and a plurality of actuators each operable to eliminate the rotor displacements according to actuator coordinates, the sensor and actuator coordinates each having a frame of reference displaced from the rotor center of gravity, a method of controlling the active magnetic bearing system to substantially eliminate the rotor displacements includes the step of transforming the sensed rotor displacements from the sensor coordinates to center of gravity coordinates, the center of gravity coordinates having the rotor""s center of gravity as a frame of reference and including at least first and second linear displacements of the rotor center of gravity and first and second rotational displacements of the rotor around the rotor center of gravity. Correction signals are generated according to predetermined gains to eliminate the first and second linear displacements and the first and second rotational displacements. The predetermined gains of the first and second rotational displacement correction signals are continuously varied in a non-linear fashion based on a determined rotational speed of the rotor. The generated correction signals are transformed from the center of gravity coordinates to the actuator coordinates. The sensed rotor displacements are then eliminated using the transformed correction signals.
In yet another embodiment of the present invention, a method of generating control signals for controlling an active magnetic bearing system to substantially eliminate lateral displacement of a rotor center of gravity from a predetermined position and a rotation of the rotor around the center of gravity, includes transforming sensed rotor displacements from sensor coordinates having a frame of reference displaced from the rotor center of gravity to center of gravity coordinates having the rotor center of gravity as a frame of reference, the center of gravity coordinates including at least first and second linear displacements of the rotor center of gravity and first and second rotational displacements of the rotor around the rotor center of gravity. Correction signals are generated according to predetermined gains for eliminating the first and second linear displacements and first and second rotational displacements. The predetermined gains of the first and second rotational correction signals are varied in a continuous non-linear fashion based on a determined rotational speed of the rotor.
In still a further embodiment of the present invention, a satellite includes a housing, a component within the housing having a rotor with a center of gravity, and an active magnetic bearing system for rotationally suspending the rotor. The active magnetic bearing system includes at least two displacement sensors, a rotational speed sensor, a controller, and at least two electromagnetic actuators. Each displacement sensor is operable to sense rotor displacements in a sensor frame of reference that is displaced from the rotor center of gravity and supply displacement signals representative thereof. The rotational speed sensor is operable to sense a rotational speed of the rotor and supply a rotational speed signal. The controller is coupled to receive the displacement signals from each of the displacement sensors and the rotational speed signal from the rotational speed sensor. The controller, in response to the received signals, is operable to transform the displacement signals from the sensor frame of reference to a center of gravity frame of reference, generate correction signals according to predetermined gains to eliminate first and second linear displacements of the rotor""s center of gravity and first and second rotational displacements around the rotor""s center of gravity, vary the predetermined gains of at least the correction signals generated to eliminate the rotational displacements in a continuous non-linear fashion based on the received rotational speed signal, and transform the correction signals to an actuator frame of reference that is displaced from the rotor center of gravity. Each electromagnetic actuator is coupled to receive the transformed correction signals and is operable, in response thereto, to eliminate the sensed rotor displacements.
Other independent features and advantages of the preferred sensor will become apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.